The present invention refers to a method for producing a mirror substrate blank of titanium-doped silica glass for EUV lithography with a thickness of at least 40 millimeters.
Furthermore, the present invention refers to a system for determining the position of defects in a mirror substrate blank of titanium-doped silica glass for EUV lithography.
EUV (Extreme Ultra Violet) lithography requires a material with no noticeable thermal expansion in the temperature range between 20° C. and 40° C. for mask and mirror substrates. Glass with a high silicic-acid content that is doped with titanium oxide, hereinafter called Ti-doped silica glass, meets these conditions. Doping with titanium oxide, however, yields a brownish coloration of the glass. The shaped articles for this kind of application, hereinafter also called blanks, are large, thick, dark-brown plates with dimensions of up to about 70×60×20 cm3, which after corresponding grinding, polishing and measuring operations, are further processed, for instance, into reflective mirrors.
What has turned out to pose problems is that, due to the manufacturing process, defects may occur in the form of bubbles or inclusions in near-surface regions of the blanks and that during polishing, these may rise to the surface of a mirror geometry and may impair the imaging quality of the mirror or mask blanks. The localization of possible defects of the blank prior to polishing is therefore a basic demand made by optics manufacturers for EUV lithography devices.
Optical measurement methods for detecting defects in the interior of glasses are normally based on an assembly in which light impinges in a vertical direction on a glass sheet and the light scattered on the defects is detected in vertical direction relative to the illumination direction. A schematic illustration of this is shown in FIG. 1b. It is thereby possible to determine the exact position of the defect point, either a bubble or an inclusion, at a distance from the surface of the glass. This measurement method is well suited for transparent glasses, but is not expedient for colored glasses that show a high light absorption. Moreover, even for transparent glasses, a limiting factor is the sample size because the light intensity (i.e., also that of the scattered light) is considerably decreasing with the path length, so that the image of a defect is no longer visible to an observer, starting from a distance defined by the lateral extension of the glass sheet. Defects located in the center of the glass sheet, i.e., at a great lateral distance from the observation position, are not accurately detected or are not detected at all.
Furthermore, it is known that defects are detected with respect to their position on the surface of opaque or semitransparent material in that a laser light is directed at an angle relative to the surface to be analyzed and the scattered light reflected by the defect is sensed by a photosensor while the material sample is moved in a controlled manner in a horizontal plane. This measurement arrangement is known, for example, from JP 02-116704. It is, however, not suited for detecting defects in the interior of the material sample and for determining the depth thereof.
WO 2006/108137 A2 suggests various systems for detecting defects in or on very thin transparent glass material for liquid crystal displays (LCD). Such glass sheets have a thickness in the range of less than one millimeter to not more than about two millimeters. The large-area glass material is passed underneath a measurement system, wherein according to one variant, the total reflection on the internal boundary surfaces of the thin glass sheet of a laser beam penetrating at an angle relative to the surface is used for detecting defects. An internally located defect generates scattered light whenever it is detected by the laser beam, which is “indirect” due to total reflection. A camera arranged at a distance from the laser light source receives this scattered light and can determine the position of the defect in horizontal direction (x/y-direction), but not in the depth (z-direction) of the thin glass sheet.
According to another variant shown in WO 2006/108137 A2, defects can be detected on both surfaces or in the interior of the thin glass sheet with respect to their position by employing the parallax shift principle with two lasers and two detectors, between which an angle is set. In consideration of the running-time measurement of the moved glass sheet, the position of the defect can also be derived within the glass sample. The measurement arrangement is complicated because one needs two respective lasers and detectors. Moreover, an exact detection of the movement speed is required for the evaluation.
DE 10 2011 087 460 B3 discloses a method for detecting defect points in a transparent body with an undefined complex surface, which method is particularly employed for checking sapphire crystals. The light irradiated into the transparent body is scattered on defect points and on the complex surface. With the help of camera systems, a series of images are recorded at several emission points of the light. As in the case of computer tomography, the course of the injected light can be reconstructed therefrom. Defect-free areas in the test pieces (sapphire crystals) can thus be identified.
According to DE 10 2009 043 001 A1, the size and shape of a defect in a transparent material can be determined, and a light beam is here incident on the specimen at the entrance area, and it runs through the specimen and is scattered on a defect. An angle-resolved measurement of scattered light caused by defects is taken at the exit area. The method is only suited for fully transparent material. Another complicated optical imaging system is required for determining the depth positions of the defects.
Similar to DE 10 2009 043 001 A1, a measured value regarding the optical quality of a transparent material is also obtained in the method according to DE 10 2004 017 237 A1.
Finally, DE 693 07 722 T2 discloses a method and an apparatus for determining defects in glasses, particularly bubbles. The measurement set-up is based on the visualization of the defects through the transparent material by image generation in the three dimensions X, Y and Z.
The standard measurement methods for localizing defects in glass material are either geared for detection on the surface, or in a very small depth range underneath the surfaces, or—as far as the matrix area is of interest—for transparent glass. In EUV lithography, however, large optics of Ti-doped silica glass are used, for the production and qualification of which the measurement methods for localizing defects in the glass material according to the prior art are inadequate.